Why trade ES if SPY is much cheaper?

Yesterday after the close I was bored and compared ES with SPY and if I look at the spread costs (buying on the Ask and immedialty selling on the Bid) and trading costs the SPY is much cheaper the ES. I have used IB for the fees

ES spread costs $12.50
ES RT $ 4.80
Total $17.30

To get the same exposure in SPY you have to trade 500 shares
SPY spread costs $ 5
SPY RT $ 5
Total $ 10

The difference is $ 7.30 per round turn / per contract. if you have 10 round turns a day you would save $ 73,-. Why should somebody trade ES or have I missed something?

I'm from Europe and don't have the Tax burden you guys have. Also PDT is not an issue for me. I understand the Tax burden but still. 7.50 per RoundTurn / Per Contract is a lot. Daytrading around 40 contracts a day would mean you save $ 300 per day. Does the Tax advantage still goes saving this amount?